Ultrashort two-section fused vertical coupler switches with high extinction ratios

We show that both cross and bar states with high extinction ratio larger than 30 dB can be achieved in ultrashort (<100 μm) fused vertical directional coupler switches with two sections by modifying the refractive indices of cores and inner cladding layers by an amount less than 1%. Transfer matrix method and the improved coupled mode theory are used to analyze these structures and the results are compared with those of 2D finite difference beam propagation method. Also, design guidelines to achieve high extinction ratios with large tolerances are presented